Multiple sclerosis (MS) is a debilitating disease of the central nervous system (CNS). The loss of the protective myelin shielding axons is the hallmark pathology associated with MS. The focal nature of the white matter plaques in the CNS of MS patients, and following experimental autoimmune encephalitis (EAE) and viral induced demyelination all support the concept that a highly evolutionarily conserved regulatory mechanism(s) prevents spread of damage to adjacent uninvolved white matter areas. However, the mechanism preventing plaque expansion undefined. A novel system is described in which viral infection in mice deficient in interleukin 10 (IL-10) results in enhanced viral clearance and reduced plaque formation. However, well after is virus is eliminated from the CNS, plaques dramatically enlarge. In contrast to other rodent models, this proposal uses a viral infection in which demyelination is independent of macrophages, IL-17, or self-reactive T cells. Importantly, the data will identify the mechanism preventing expansion of focal demyelination and directly tests the emerging concept of active communication between astrocytes and microglia in regulating demyelination. This model system will allow definition of the cells secreting IL-10 and the cell which receives the signal restraining focal myelin injury. Preliminary data indicate that Foxp3+ regulatory T cells (Treg) are the source of IL-10 limiting demyelination. The first aim will be accomplished by specific depletion of Treg from the CNS by infusion of diphtheria toxin using an osmotic pump approach to supply toxin intraventricularly after resolution of acute infection and confirmed by ablation of IL-10 secretion only from Treg. The second Aim will define the cell within the CNS which is the recipient of the IL-10 signaling that limit tissue damage. This will be accomplished by infecting mice in which the IL-10 receptor has been specific ablated in astrocytes, the only cell type expressing the receptor during chronic demyelination. Demyelination in this model is independent of bone marrow derived macrophages, therefore gene profiling of cells purified from the infected adult CNS will be used to define the interaction of IL-10 with astrocytes and the subsequent alteration in microglia activity. These data will provide the first evidence for a temporally distinct pathological event within the CNS that is vira induced, but distinct from the acute event and demonstrate that limiting the expansion of lesions within white matter are dependent upon the astrocyte response to the anti-inflammatory cytokine IL-10.